1. Field of Invention
The present invention relates generally to golf clubs and, more particularly, to golf club shafts.
2. Description of the Related Art
Over the years, many substitutes have been introduced for the hard wood shafts originally used in golf club drivers and irons. Early substitute materials included stainless steel and aluminum. More recently, carbon fiber reinforced resin shafts have become popular. Fiber reinforced resin shafts are typically hollow and consist of a shaft wall formed around a tapered mandrel. The mandrel typically consists of three mandrel sections. The first mandrel section forms the tip section of the shaft, the second mandrel section forms the main body section, and the third mandrel section forms the grip section. As shown in FIG. 1, shafts formed in this manner typically have a constant taper from the tip/main body intersection to the main body/grip intersection. In other words, the taper of the main body section is constant. Additionally, in order to reduce the weight of the shaft, the shaft wall thickness in conventional shafts tends to decrease uniformly (i.e. at a constant rate without abrupt changes), at least from the tip/main body intersection to the main body/grip intersection.
The use of fiber reinforced resin has allowed golf club manufacturers to produce shafts having varying degrees of torsional and longitudinal stiffness to satisfy the needs of a wide variety of golfers. Torsional stiffness relates to a golf club's ability to resist twisting along its length when a golf ball is struck. The inertia of the ball produces a force on the head tending to rotate the head about the axis of the shaft relative to the grip section. Longitudinal stiffness refers to a golf club's ability to resist bending when subjected to a force.
For a given grip outer diameter (OD) and a given tip OD, the conventional method of increasing the torsional and longitudinal stiffness of a fiber reinforced resin shaft is to increase the thickness of the shaft wall. However, because the fiber reinforced resins used to make the shaft are expensive, the use of additional material to increase the shaft wall thickness raises the cost of the shaft to an undesirable level. Additionally, increasing the shaft wall thickness adds weight to the shaft, which is also undesirable. Another method of increasing torsional and longitudinal stiffness is to use materials with a higher modulous of stiffness. Due to the higher cost of these materials, this method is also undesirable.
More recently, so-called "bubble" or "wide body" shafts have been introduced in an attempt to increase shaft stiffness. Here, the OD of selected portions of the main body section is greater than that of a shaft having a constant taper from the tip section to grip section. In some cases, the main body section's proximal portion (the portion of the main body section closest to the golfer) will have a greater OD than the distal end of the grip section (the end of the grip section farthest from the golfer). As such, the main body section of certain "bubble" or "wide body" shafts includes a short connecting portion which rapidly decreases in diameter, thereby connecting the main body section to the grip section. Such a shaft is disclosed in U.S. Pat. No. 5,316,299 to Feche et al. The shaft stiffness is generally greater in "bubble" or "wide body" shafts than in conventional shafts formed with the same materials and having the same wall thickness. Nevertheless, there are limitations with respect to the maximum OD of the main body section. Accordingly, there is a need for additional methods of increasing shaft stiffness that do not substantially increase the weight or cost of the shaft.